This invention relates to electric motors of the type having brushes for commutation. More particularly, this invention relates to a brush/shunt assembly having a stranded shunt and to a method of connecting the brush shunt to a termination point on an electric motor.
It is well-known that electric motors typically include a stationary stator portion and a rotor or armature mounted for rotation within the stator. The stationary stator portion includes a stator core in which the rotor rotates and may include other stationary parts of the motor such as a frame that holds the ends of the rotor shaft. The stator core is either a permanent magnet, which might be a ceramic material, or a material, such as a metal, capable of being magnetized. If the stator core is metallic, it typically includes one or more coils ("field coils" or "stator coils") wound around the stator core that transform the stator core into an electromagnet when connected to a source of electrical energy. The rotor also includes one or more coils ("armature coils" or "rotor coils") that produce a magnetic field in the rotor core when electrical energy is applied. The rotor rotates as the magnetic fields in the stator core and the rotor core repel one another.
In order to conduct electrical energy to the rotor coils, a commutator is mounted around the rotor shaft. The commutator includes two or more commutator bars or rings extending circumferentially around the shaft. Brushes secured to the stator core, or to a support structure attached to the stator core, make sliding contact with the commutator as the rotor rotates.
In many types of motors, it is necessary to use a conductor known as a shunt to connect each brush to a termination point on the stationary stator portion, which is also connected either to one of the stator coils or to a source of electrical energy, or both (depending on whether, e.g., the motor is a series-type motor or a shunt-type motor, or a permanent magnet motor as opposed to a motor having stator windings).
Brushes are typically urged--e.g., by springs--toward the commutator so that contact is maintained as the brush wears down. In order for the shunt to be sufficiently flexible so as not to interfere with movement of the brush under the action of the spring, the shunt is frequently made from fine stranded wire, which may be braided or twisted.
One type of termination that is commonly used to make connections on a stator assembly is a terminal having a slot between two arms that are urged toward one another. A wire is inserted in the slot and is held in place by the arms. The arms also have sharp edges which cut into the surface of the wire, both to cut through any insulation or insulating coating (which is common on wire used for coil windings) and to assure good mechanical and electrical contact. Heretofore, it has not been possible to use such terminals with uninsulated stranded wire, such as brush shunts, because the sharp edges would cut through the strands and the shunt would fail, or at best would achieve poor electrical contact.
As a result, when stranded wire shunts are used in electric motors, it is common to terminate the stator coil to a slotted terminal and then provide a separate terminal, electrically connected to the slotted terminal, for connecting the shunt. For example, a one-piece terminal having a male quick-disconnect tab on one end and a slotted termination portion on the other end could be mounted on the stator assembly. The stator coil would be electrically terminated to the slotted terminal portion. A female quick-disconnect terminal could be-connected to the end of the shunt, as by crimping, soldering or both. The male and female quick-disconnect terminals are then mated to one another. This requires providing (1) an additional terminal on the shunt, (2) a crimping or soldering step, and (3) a terminal mating step. The connection of the shunt to the stator thus becomes time-, labor- and material-intensive.
In addition, if accessory components are to be electrically connected to the motor circuit--e.g., if a light bulb for use as a headlight in a powered vacuum cleaner nozzle is to be connected in parallel with the armature coils of the nozzle motor--it is common to mount an additional terminal on the end of each accessory lead and a mating terminal, electrically connected to the shunt terminal, on the stator assembly. Alternatively, a two-into-one adaptor can be attached to the existing stator terminal, to which separate terminals on the ends of the shunt and the accessory lead are both mated. Either approach adds to the cost in time, labor and material.
It would be desirable to be able to provide a way to quickly and easily connect a stranded brush shunt wire to a terminal on a stator assembly.
It would also be desirable to be able to minimize the number of terminals needed on a stator assembly to accomplish all of the necessary stator connections.